Generally, an offshore jacket is comprised of at least three substantially vertical legs that are interconnected by framing or cross-bracing members to form a triangular or rectangular base, wherein a leg is disposed at each corner of the base. In its upright position, the jacket rest on the sea floor with the bottom of the legs resting on the sea floor or slightly penetrating into the soil. The jacket is secured to the sea floor with piles which are either driven through the legs or driven through sleeves attached to the legs. FIG. 1 shows a traditional offshore jacket. The flared jacket 25 provides wider base (b) for greater stability when attached to the sea floor.
In many areas of the world, the soil of the sea floor is unconsolidated and very soft resulting in very low allowable bearing pressures. These soft sea floors occur frequently near the mouths of large rivers that empty into the oceans. Sea beds in the world which exhibit high hydrocarbon content but are characterized by soft soils from river deltas include areas in the Gulf of Mexico, west Africa and southeast Asia.
The low bearing pressures of these unconsolidated sea floors create jacket support problems during installation of offshore platforms. Specifically, without adequate support, the legs of a jacket will sink into the sea floor, causing the jacket to either fall onto its side or settle lower than design specifications. In any case, jacket settling due to a soft sea floor can negatively affect the alignment of the jacket as it is positioned at the drilling site. In this same vein, difficulties often arise during pile driving operations, which are generally completed within one to two weeks of placing a jacket in position on the sea floor.
One solution to the difficulties associated with unconsolidated sea floors is to provide a structure that spreads the downward forces applied to the jacket over a larger area of the sea floor. The most common structure for accomplishing this task is called a mudmat. A mudmat has a very large surface area that rests against the sea floor (as opposed to the comparatively small surface area of a jacket leg), distributing the load of the jacket over a larger sea floor, thus allowing the jacket to properly stand on the soft sea floor and to provide stability during pile-driving operations. The bearing plate rests against the sea floor and provides the large surface area for force distribution.
There are several different types of units. Of course one of the first developed was the fixed platform in which the legs or supports of the rig are permanently installed, penetrating the floor of the body of water in which the well is to be drilled as discussed previously in FIG. 1. Such a structure is limited by water depth and does not provide the mobility and flexibility of the mobile or portable type unit.
One form of unit is the self-elevating platform, sometimes called “bootstrap” or “jack-up”, units which are moved to a use site. These units with a plurality of legs, usually three, are lowered from a floating platform through the water for engaging sea floor The footings (cleats or feet), engage with the sea floor, then the platform is jacked up a sufficient distance above the water surface to get the platform above the wave action. U.S. Pat. Nos. 3,996,754 to Lowery and 4,265,568 to Herrmann et al. are representative of this type. Although such units are highly mobile and stable when in place, they are less stable when floating and when in transit from site to site and are limited to a range of water depth while the unit is afloat. In areas of extreme weather conditions, the three or more legs of such rigs may not have the required stability as the base (b) is limited by the size of the platform. This example is shown in FIG. 2. FIG. 2 shows platform 10 supported by three legs 20, connected to cleats or feet 50 on the sea floor 1. The cleats or feet 50 are symmetric with respect to the legs such that the center of pressure 51 exerted by the sea floor 1 is congruent with the center of the legs 20. While those cleats or feet 50 are shown as octagonal, many other symmetric shapes are commonly used, circles, squares, rectangles, ovals, etc. However, each shape is symmetric with respect to the leg to ensure the center of pressure is under the leg, and external bending moments on the legs 20 are minimized.
The present subject matter provides the mobility, low cost and stability in a self-elevating type unit by extending the base beyond the traditional limits of self-elevating platforms, enabling compact transportation and a distributing leg reactions over a larger base (b′) on the sea floor.
These and many other advantages of the present subject matter will be readily apparent to one skilled in the art to which the invention pertains from a perusal of the claims, the appended drawings, and the following detailed description of preferred embodiments.